The present invention relates generally to methods and apparatus for reproducing images and alphanumeric characters, and more particularly to a thermal inkjet, multi-nozzle drop generator, printhead construction, and its method of operation.
The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ inkjet technology for producing hard copy printed output. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (Oct. 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Inkjet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
The quality of a printed image has many aspects. When the printed matter is an image, it is the goal of a printing system is to accurately reproduce the appearance of the original. To achieve this goal, the system must accurately reproduce both the perceived colors (hues) and the perceived relative luminance ratios (tones) of the original. Human visual perception quickly adjusts to wide variations in luminance levels, from dark shadows to bright highlights. Between these extremes, perception tends toward an expectation of smooth transitions in luminance. Printing devices and similar imaging systems generally create an output that reflects light to provide a visually observable image. Exceptions such as transparencies exist, of course, but for consistency, the term reflectance will be used to denote the optical brightness of the printed output from a printing device. Generally speaking, reflectance is a ratio of the light reflected from a surface to that incident upon it. The colorants deposited upon the medium by inkjet printers are usually considered to be absorbers of particular wavelengths of light energy. This selective absorption prevents selected wavelengths of the light energy incident upon the medium from reflecting from the medium and is perceived by humans as color. Printing systems have yet to achieve complete and faithful reproduction of the full dynamic range and perception continuity of the human visual system. While it is a goal to achieve the quality of photographic image reproduction, printing dynamic range capabilities are limited by the sensitivity and saturation level limitations inherent to the recording mechanism, although the effective dynamic range can be extended somewhat by utilizing non-linear conversions that allow some shadow and highlight detail to remain.
An inkjet printer for inkjet printing typically includes a print cartridge in which small drops of ink are formed and ejected towards a print medium. Such cartridges include a printhead having an orifice member or plate that has a plurality of small nozzles through which ink drops are ejected. Adjacent to the nozzles are ink-firing chambers, where ink resides prior to ejection through the nozzle. Ink is delivered to the ink-firing chambers through ink channels that are in fluid communication with an ink supply, which may be contained in a reservoir portion of the pen or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink firing chamber by selectively energizing a heater resistor positioned in the ink firing chamber. This thermal process causes ink within the chamber to vaporize and form a vapor bubble. The rapid expansion of the bubble forces ink through the nozzle.
Once ink is ejected, the ink-firing chamber is refilled with ink from the ink channel. This ink channel is typically sized to refill the ink chamber quickly to maximize print speed. Ink channel damping is sometimes provided to dampen or control inertia of the moving ink flowing into and out of the firing chamber. By damping the ink flow between the ink channel and the firing chamber, the oscillatory underfilling and overfilling of the firing chamber and the resulting meniscus recoiling and bulging from the external orifice of the nozzle, respectively, can be avoided or minimized.
As the vapor bubble expands within the firing chamber the expanding vapor bubble can extend into the ink channel in a detrimental action known as "blowback". Blowback tends to result in forcing ink in the ink channel away from the firing chamber. The volume of ink which the bubble displaces is accounted for by both the ink ejected through the nozzle and ink which is forced down the ink channel away from the firing chamber. Therefore, blowback increases the amount of energy necessary for ejecting droplets of a given size from the firing chamber. The energy required to eject a drop of a given size is referred to as "turn on energy". Printheads having high turn-on energies tend to be less efficient and therefore, have more heat to dissipate than lower turn-on energy printheads. Assuming a fixed capacity to dissipate heat, printheads that have a higher thermal efficiency are capable of a higher printing speed or printing frequency than printheads that have a lower thermal efficiency.
Following removal of electrical power from the heater resistor, the vapor bubble collapses in the firing chamber. Components within the printhead in the vicinity of the vapor bubble collapse are susceptible to cavitation stresses as the vapor bubble collapses between firing intervals. The heater resistor is particularly susceptible to damage from cavitation. A hard thin protective passivation layer is typically applied over the resistor to protect the resistor from stresses resulting from cavitation. The passivation layer, however, tends to increase the turn-on energy required for ejecting droplets of a given size.
In inkjet technology, which uses dot matrix manipulation to form both images and alphanumeric characters, the colors and tone of a printed image are modulated by the presence or absence of drops of ink deposited on the print medium at each target picture element (known as a "pixel") generally represented as a superimposed rectangular grid overlay of the image. The medium reflectance continuity--tonal transitions within the recorded image on the medium--is especially affected by the inherent quantization effects of using quanta of ink drops and dot matrix imaging. These quantization effects can appear as a contouring in a printed image where the original image had smooth transitions. Moreover the printing system can introduce random or systematic reflectance fluctuations or graininess which is the visual recognition of individual or clusters of dots with the naked eye.
Perceived quantization effects which detract from print quality can be reduced by decreasing the density quanta at each pixel location in the imaging system and by utilizing techniques that exploit the psycho-physical characteristics of the human visual system to minimize the human perception of the quantization effects. It has been estimated that the unaided human visual system will perceive individual ink dots until they have been reduced to approximately twenty-five microns in diameter or less on in the printed image. Therefore, undesirable quantization effects of the dot matrix printing method have been reduced by decreasing the size of each drop and printing at a high resolution; that is, a true 1200 dots per inch ("dpi") placement of small dots on a printed image looks better to the eye than a true 600 dpi image of larger dots, which in turn improves upon 300 dpi of even larger dots, etc. Additionally, undesired quantization effect can be reduced by utilizing more colors with varying densities of color (e.g., two cyan ink print cartridges, each containing a different ratio of dye to solvent in the chemical composition of the ink) or containing different types of chemical colorants.
To reduce quantization noise effects, print quality also can be enhanced by firing multiple drops of the same color or color formulation at each pixel resulting in more "levels" per color and reducing quantization noise. Such methods are discussed in U.S. Pat. No. 4,967,203 to Alpha N. Doan et al. for an "Interlace Printing Process", U.S. Pat. No. 4,999,646 to Jeffrey L. Trask for a "Method for Enhancing the Uniformity and Consistency of Dot Formation Produced by Color Ink Jet Printing", and U.S. Pat. No. 5,583,550 to Mark S. Hickman et al. for "Ink Drop Placement for Improved Imaging" (each assigned to the assignee of the present invention).
One can also reduce graininess in a picture by essentially low pass filtering the printed image with smoothing techniques that decrease resolution but, importantly, reduce noise. One such technique dilutes the ink (by one-fourth the original optical density by adding three parts solvent) such that the ink drop which would have been deposited on a single pixel (in, for example, a 600 dpi resolution) is spread over at least portions of adjacent pixel areas. While each drop would contain the same amount of colorant, the additional solvent causes the colorant to be distributed over a wider area. As stated, this lowers the visual noise at the cost of perceived resolution. Additionally, this technique places substantially more solvent on the printed medium resulting in an unacceptably long time to dry, consumes much more ink for printing, and slows down the speed of printing
In multiple drop modes of printing, the resulting dots vary in size or in color depending on the number of drops deposited in an individual pixel and the constitution of the ink with respect to its spreading characteristics after impact on the particular medium being printed (plain paper, glossy paper, transparency, etc.). The reflectance and color of the printed image on the medium is modulated by manipulating the size and densities of drops of each color at each target pixel. The quantization effects of this mode can be reduced in the same ways as for the single-drop per pixel mode. The quantization levels can also be reduced at the same printing resolution by increasing the number of drops that can be fired at one time from nozzles in a printhead array and either adjusting the density of the ink or the size of each drop fired so as to achieve full dot density. However, simultaneously decreasing drop size and increasing the printing resolution, or increasing the number of cartridges and varieties of inks employed is expensive, so older implementations of inkjet printers designed specifically for imaging art reproduction generally use multi-drop modes or multiple passes to improve color saturation.
When the size of the printed dots is modulated, the image quality is very dependent on dot placement accuracy and resolution. Misplaced dots leave unmarked pixels which appear as white dots or even bands of white lines within or between print swaths (known as "banding"). Mechanical tolerances become increasingly critical in the construction as the printhead geometries of the nozzles are reduced in order to achieve a resolution of true 600 dpi or greater. Therefore, the cost of manufacture increases with the increase of the resolution design specification. Furthermore, as the number of drops fired at one time by multiplexing nozzles increases, the minimum nozzle drop volume decreases, dot placement precision requirements increase. Also the thermal efficiency of the printhead becomes low, leading to high printhead temperatures. High printhead temperatures can lead to reliability problems, including ink out-gassing, erratic drop velocities due to inconsistent bubble nucleation, and variable drop weight due to ink viscosity changes. Moreover, when the density of the printed dots is modulated as in multi-dye load ink systems, the low dye load inks require that more ink be placed on the print media, resulting in less efficient ink usage and higher risk of ink coalescence and smearing. Ink usage efficiency decreases and risk of coalescence and smearing increases with the number of drops fired at one time from the nozzles of the printhead array.
Smaller drops naturally suggest smaller nozzles. As the nozzle area is made smaller, the nozzle becomes more susceptible to plugging by solid contaminants in the ink or by particles created in the process of manufacturing the print cartridge. Additionally, the smaller nozzles require a thinner orifice plate as the size of the entire drop generator mechanism is made smaller.
In light of the foregoing, it is desirable to obtain an inkjet printhead and printing system in which small drops are reliably expelled and deposited upon a print medium in such a manner that a high degree of visual dynamic range concurrent with reduced quantization and granularity.